Search Results for (Other, general, or unspecified) processing-related failures

An elbow of 70 mm OD and 10 mm wall thickness made from St 35.29, and exposed to 315 atmospheres internal pressure in an oil hydraulic shear installation, cracked lengthwise after a short operating period. Because the stress was not sufficient to explain the fracture of this elbow under this pressure, an investigation was conducted to establish whether material or processing errors had occurred. Microscopic examination showed that a ferritic-pearlitic structure in select locations was very fine-grained. Other signs of fast cooling as compared to normally formed structure of the core zone were noted. It was also possible that the pipe was resting on a cold plate during bending or that it came in touch with a cold tool. This apparently caused the strains at the transition to the cross-sectional part that had been cooled more slowly. The location of the crack at just this point gave rise to the conclusion that it was formed either by the sole or contributive effect of these stresses.

Occasional failures were experienced in spool-type valves used in a hydraulic system. When a valve would fail, the close-fitting rotary valve would seize, causing loss of flow control of the hydraulic oil. The rotating spool in the valve was made of 8620 steel and was gas carburized. The cylinder in which the spool fitted was made of 1117 steel, also gas carburized. Investigation (visual inspection, low magnification images, 400x images, metallographic exam, and hardness testing) supported the conclusion that momentary sliding contact between the spool and the cylinder wall caused unstable retained austenite in the failed cylinder to transform to martensite. The increase in volume resulted in sufficient size distortion to cause interference between the cylinder and the spool, seizing, and loss of flow control. The failed parts had been carburized in a process in which the carbon potential was too high, which resulted in a microstructure having excessive retained austenite after heat treatment. Recommendations included modifying the composition of the carburizing atmosphere to yield carburized parts that did not retain significant amounts of austenite when they were heat treated.

A forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

A cadmium-plated 4340 Ni-Cr-Mo steel ballast elbow assembly was submitted for failure analysis to determine the element or radical present in an oxidation product found inside the elbow assembly. Energy-dispersive x-ray analysis in the SEM showed that iron was the predominant species, presumably in an oxide form. The inside surface had the appearance of typical corrosion products. Hardness measurements indicated that the 4340 steel was heat treated to a strength of approximately 862 MPa (125 ksi). It was concluded that the oxide detected on the ballast elbow was iron oxide. The possibility that the corrosion products would eventually create a blockage of the affected hole was great considering the small hole diameter (4.2 mm, or 0.165 in.). It was recommended that a quick fix to stop the corrosion would be to apply a corrosion inhibitor inside the hole. This, however, would cause the possibility of inhibitor buildup and the eventual clogging of the hole. A change in the manufacturing process to include a cadmium plating on the hole inside surface was recommended. This was to be accomplished in accordance with MIL specification QQ-P-416, Type II, Class 1. A material change to 300-series stainless steel was also recommended.

Cracking was found in the heads on large Yankee dryers, large, cylindrical, rotating, pressurized, high-temperature, cast iron pressure vessels (ASME Boiler and Pressure Vessel Code Section VIII, Rules for Construction of Pressure Vessels), used to remove moisture from sheets of tissue paper during manufacturing. The typical components consist of a cast iron shell, two cast iron concave heads, and a large cast iron internal center stay attached to journals. The heads are attached to the shell and center stay with high-strength bolts. FEA and metallurgical investigation supported the conclusion that the cracking was caused by an unexpected type of load placed on the machine, namely corrosion product buildup at the head/shell interface causing the joint to displace open. It was also found that compressive bolting loads could slightly open the head/shell interface at the periphery. Recommendations included design changes in the head/shell joint, and detailed preventive maintenance inspection procedures were also suggested.

The handle from a consumer product exhibited an apparent surface defect. The handle had been injection molded from a medium viscosity grade ABS resin. The anomalous appearance was objectionable to the assembler of the final product and resulted in a production lot being placed on quality-control hold. Investigation included visual inspection, 24x micrographs, and FTIR in the reflectance mode. The spectrum obtained on the included material was characteristic of polybutadiene, the rubber-modifying agent present in ABS. This supported the conclusion that the inclusion's most likely source was an undispersed gel particle formed during the production of the molding resin.

A section of clear polymeric tubing failed while in service. The failed sample had been used in a chemical transport application. The tubing had also been exposed to periods of elevated temperature as part of the operation. The tubing was specified to be a polyvinyl chloride (PVC) resin plasticized with trioctyl trimellitate. Investigation included visual inspection, micro-FTIR in the ATR mode, and thermogravimetric analysis. The spectrum on the failed tubing exhibited absorption bands indicative of a PVC resin containing an adipate-based plasticizer. Thermograms of the failed pieces and a reference sample of tubing that performed well showed that the reference material contained a trimellitate-based plasticizer and that the failed material contained an adipate-based material. The conclusion was that the failed tubing had been produced from a formulation that did not comply with the specified material and, as a result, was not as thermally stable as the reference material.

Molded plastic couplings used in an industrial application exhibited abnormally brittle properties, as compared to previously produced components. The couplings were specified to be molded from a custom-compounded glass-filled nylon 6/12 resin. An inspection of the molding resin used to produce the discrepant parts revealed that the pellets were of two general types, neither of which matched the pellets from a retained resin lot. Investigation included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The thermograms supported the conclusion that the brittle couplings contained a significant level of contamination, polypropylene and nylon 6/6. The source of the polypropylene was likely the purging compound used to clean the compounding extruder. The origin of the nylon 6/6 resin was unknown but may represent a previously compounded resin.

A stamped coin exhibited visible discolored areas, seen as a tan haze on the surface. The discoloration was considered merely cosmetic. The nonstained and stained regions were studied using SEM/EDS. Greater amounts of aluminum and magnesium were found in the stained area as compared with the nonstained region. Some carbon and oxygen were detected in both areas, which may be suggestive of organic substances. Fourier transform infrared spectroscopy (FTIR) revealed traces of hydrocarbons and ether/alcohol materials in the stained area, suggesting that the stain was associated with a cellulose or carbohydrates (sugars). These findings, along with the appearance, suggest that a sugar-containing substance, such as coffee or a soft drink, dried onto the surface of this coin and caused the observed discoloration.

A production lot of plastic wire clips was failing after limited service. The failures were characterized by excessive relaxation of the clips, such that the corresponding wires were no longer adequately secured in the parts. No catastrophic failures had been encountered. Parts representing an older lot, which exhibited satisfactory performance properties, were also available for reference purposes. The clips were specified to be injection molded from an impact-modified grade of nylon 6/6. However, the part drawing did not indicate a specific resin. Investigation included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The spectrum representing the reference parts showed a relatively higher level of a hydrocarbon-based impact modifier, while the results obtained on the failed parts showed the presence of an acrylic-based modifier. Also, the reference clip thermogram showed a melting transition attributed to a hydrocarbon-based impact modifier. The conclusion was that the control and failed clips had been produced from two distinctly different resins. It appeared that the material used to produce the failed clips had different viscoelastic properties, which produced a greater predisposition for stress relaxation.

A drawing plant which processed steel wire of designation 105 Cr 2 for ball bearings had losses due to crack formation and wire breakage during drawing. To establish the reason for the breakage, seven fractures were submitted for investigation with contiguous wire segments on both sides of the fracture of 300 mm each. Missing in the lamellar surface structure, with the exception of the remnants of a coarse network, were the pre-eutectically precipitated carbides to be expected in this steel. Surrounding the ferritic region in the surface structure, a ring of lamellar pearlite is seen, which turns into the granular annealed structure towards the core. The described structural phenomena were noted in all of the seven fracture regions. Their intensity always decreased with increasing distance from the fracture. Surface decarburization caused the formation of lamellar pearlite during annealing. This investigation further revealed that the localized decarburization and pearlite formation was present already in the rolled wire in uneven distribution over the entire coil length.

Splined rotor shafts (constructed from 1151 steel) used on small electric motors were found to miss one spline each from several shafts before the motors were put into service. Apparent peeling of splines on the induction-hardened end of each rotor shaft was revealed by visual and stereo-microscopic examination. One tooth on each shaft was found to be broken off. It was revealed by metallographic examination of an unetched section through the fractured tooth that the fracture surface was concave and had an appearance characteristic of a seam. Partial decarburization of the surface was revealed after etching with 1% nital. The presence of a crack, with typical oxides found in seams at its root, was disclosed by an unetched section through the shaft in an area unaffected by induction heating. The etched samples revealed similar decarburization as was noted on the fracture surface of the tooth. It was concluded that the seam had been present before the shaft was heat treated and these seams acted as stress raisers during induction hardening to cause the shaft failure. It was recommended that the specifications should specify that the shaft material should be free of seams and other surface imperfections.

One percent Cr-Mo low alloy constructional steel is widely used for high tensile applications, e.g., for manufacture of high tensile fasteners, heat treated shafts and axles, for automobile applications such as track pins for high duty tracked vehicles etc. The steel is fairly through hardening and heat treatment does not present any serious difficulty. Care is still required in processing to avoid decarburization. In an application of track pins for tracked vehicles, bars about 22 mm diam were required in heat treated and centerless-ground condition prior to induction hardening of the surface. Indifferent results were obtained in induction hardening; cracks were noticed, and patchy hardness figures were obtained on the final product in several batches. Metallographic examination of transverse sections through the defective areas showed decarburization to varying degrees, i.e., from partial to total decarburization. Observations suggested the defects originated at the stages of ingot making and rolling. This was apparently the reason for complete decarburization of the area with original surface defect which opened up further in the oxidizing atmosphere of the furnace with low melting clinkers from scale and furnace lining filling up the crevice of the original defect.

Cracking occurred within the plastic jacket (injection molded from an impact-modified, 15% glass-fiber-reinforced PET resin.) of several assemblies used in a transportation application during an engineering testing regimen which involved cyclic thermal shock (exposing the parts to alternating temperatures of -40 and 180 deg C (-40 and 360 deg F)). Prior to molding, the resin had reportedly been dried at 135 deg C (275 deg F). The drying process usually lasted 6 h, but occasionally, the material was dried overnight. Comparison investigation (visual inspection, 20x SEM views, micro-FTIR, and analysis using DSC and TGA) with non-failed parts supported the conclusion that that the failure was via brittle fracture associated with the exertion of stresses that exceeded the strength of the resin as-molded caused by the disparity in the CTEs of the PET jacket and the mating steel sleeve. The drying process had exposed the resin to relatively high temperatures, which caused substantial molecular degradation, thus limiting the part's ability to withstand the stresses. The drying temperature was found to be significantly higher than the recommendation for the PET resin, and the testing itself exposed the parts to temperatures above the recognized limits for PET.

To permit bolting of a 90 lb/yd. flat-bottomed rail to a steel structure, rectangular slots 2 in. wide x 1 in. deep were flame-cut in the base of the rail at 2 ft intervals to suit existing bolt holes. During subsequent handling, one of the rails (which were about 25 ft long) was dropped from a height of approximately 6 ft on to a concrete floor and it fractured into 11 pieces, each break occurring at a slot. The sample piece submitted for examination showed a wholly brittle fracture at each end, the fractures having originated at the sharp corners of the slots. During flame-cutting, a narrow band of material on each side of the cut was raised above the hardening temperature. When the torch had passed the rate of abstraction of heat from this zone by conduction into the cold mass of the rail was sufficiently rapid to amount to a quench and thus cause local hardening. The steel in the regions of the slots possessed little capacity for deformation, and fracturing of the martensitic layer, under cooling or impact stresses, would be likely to occur. The slots should have been cut mechanically.

There was a large incidence of surface defects on the crank pins and journals and other areas of crank shafts of a high power automotive engine. The steel used was a Cr-Mo type of nitriding steel. Metallographic observations conclusively proved that the defective areas were entrapment of foreign bodies, resulting from steel making/deoxidizing/teeming stages. The occasionally globular nature of the foreign particles suggested these were formed at the liquid condition of the steel. The ratio of Mn-Si as seen on electron probe microanalysis also suggested the globules high in Mn content might have resulted in deoxidizing stage. Particularly the absence of Fe in some areas in the inclusion was indicative of precipitation deoxidation by ferromanganese/ferrosilicon. The defects apparently did not have time to coalesce and rise up to the top.

A ceiling in a concrete structure was hung on flat bars with a cross section of 30 x 80 mm. The bars were borne by a slit steel plate and supported by tabs that were welded onto the flat sides. One of the bars fractured during mounting when it was dropped from a height of about 1 m onto the opposite support. The fracture was a grainy forced rupture that propagated from one of the fillet welds. Investigation showed a steel was selected for this important construction that was prone to aging and that in fact had aged through cold deformation during straightening and then was welded yet. The bar could withstand mounting and subsequent static loading as long as it was treated with care, as could be expected from the good deformation characteristics of the static tensile test. The question is, however, whether occasional impacts or shocks can be assuredly avoided. This risk could have been eliminated if a killed steel of quality groups 2 or 3 according to DIN 17 100 had been used.

A 25 mm (1 in.) diam carrier shaft failed suddenly during operation. The shaft failed near the toe of the 4.8 mm (316 in.) frame-to-shaft 60 deg and 120 deg submerged metal arc (SMA) tack welds after an unknown time in service. Material specifications called for the shaft to be made from SAE 1018 cold-rolled carbon steel. Carrier assembly components were made from type 300 stainless steel, and all nuts, spacers, and washers were to be SMA tack welded to the stainless steel frame. Chemical analyses (OES, SEM/EDS) showed the shaft to actually be made from SAE 1050 high-carbon steel and that a low-carbon steel welding procedure had been used. This resulted in incipient cracks in the stainless steel weld metal near the toes of the component-to-shaft welds. The hardnesses of the heat-affected zones were as high as 58 HRC, and they were grain coarsened. The parting of the shaft was determined to have been caused by an impact failure mechanism, with the origin at the incipient cracks in the weld metal. Additionally, the coarsened heat-affected zones were found to be hydrogen embrittled. The primary cause of the failure was the use of an unspecified material.

An aircraft engine in which an in-flight fire had occurred was dismantled and examined. A bracket assembly fabricated from 2024 aluminum, one of several failed components, was of prime interest because of apparent heat damage. Scanning electron microscopy was used to compare laboratory-induced fractures made at room and elevated temperatures with the bracket failure. The service failure exhibited grain separation and loss of delineation of the grain boundaries due to melting. SEM revealed deep voids between grains and tendrils that connected grains, which resulted from surface tension during melting. Microscopic examination of polished, etched section through the fractured surface verified intergranular separation and breakdown of grain facets. The absence of any reduction of thickness on the bracket assembly at the point of fracture, along with evidence of intense heat at this point, indicated that little stress had been applied to the part. Comparisons of the service failure and laboratory-induced failures in conjunction with macroscopic and metallographic observations showed that the bracket assembly failed because an intense, localized flame had melted the material.

Examples of metallic inclusions in steels of various types are presented. The structure of an inclusion in an annealed Fe-1C-1.5Cr steel consisted of ferrite with lamellar pearlite. The carbon content of the inclusion was therefore considerably lower than that of the chromium steel and was adapted to the latter by diffusion only at the periphery of the inclusion. In another section of a hardened piece of the same chromium steel, the steel in this case had a structure of martensite with hypereutectic carbide, while the inclusions consisted of a very fine laminated eutectoid of the lower pearlite range (Troostite). In a pipe of 18-8 austenitic stainless steel a weakly magnetizable spot of limited size was found. This inclusion too was probably more alloy-deficient than the austenitic steel, similar to the ones described above. All three cases were casting defects.